The SEC might argue that bar patrons have such a duty of confidence - they have previously done so for roommates, and golfing partners. Or they might charge the person you overheard with violating the Reg FD selective disclosure rules. Or they might let you both away with it (IANAL). But yes, it absolutely is part of the law; http://www.law.cornell.edu/cfr/text/17/240.10b5-2 is the detailed version.